File:Dust plume and Gravity Waves off Sudan (MODIS).jpg

English: In mid-July 2023, a thick plume of light tan dust blew over the coast of Sudan and across the Red Sea.

On July 20, the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired a true-color image of the dust storm, showing the source of the dust appeared to be sediment from the delta of the Baraka River, near the town of Tokar. Because of the position of the MODIS instrument, the image also captures two other phenomena: sunglint and gravity waves.

When the satellite passed over the Red Sea, it was positioned so that sunlight reflected off the blue waters at the exact same angle that the satellite sensor viewed it. As a result, the Red Sea took on a mirror-like sheen, called “sunglint” wherever the water was smooth. In areas of roughened water, the sunlight no longer reflects back at the sensor, so the water appears dark.

A group of parallel waves can be seen as dark lines in the Red Sea just off Sudan’s shore. This wave pattern is created by atmospheric gravity waves on the surface of the ocean, and is visible only because of the sunglint. As the name implies, atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down. On its descent into the low-point of the wave (the trough), the air touches the surface of the water, roughening it. The long, vertical dark lines show where the troughs of gravity waves have roughened the surface. The brighter regions show the crests of the atmospheric waves. Beneath the crests, the water is calm and reflects light directly back towards the sensor.